Generally, construction machines like hydraulic excavator are mounted on a crawler type lower structure with right and left crawler belts to drive the machine in a stable state even on rough terrains such as mountainous and muddy terrains.
The conventional crawler type lower structures of this sort are provided with a truck frame including a center frame for supporting an upper structure, right and left side frames which are located on the right and left sides of the center frame to extend toward and between front and rear ends, and front and hind legs which are bridged between the center frame and the right and left side frames (e.g., as disclosed in Japanese Laid-Open Patent Application H9-142337).
A center frame is providing on the round girth member, and four legs, i.e., a left fore leg, a right fore leg, a left hind leg and a right hind leg, are extended radially outward in the shape of letter “X” from the center frame to connect the center frame with the right and left side frames. In this instance, for example, a left fore leg is formed in a box-like structure enclosed by upper and lower leg-forming plates which are extended in face to face relation from the center frame toward the left side frame, and by front and rear leg-forming plates which are welded to the front and rear edges of the upper and lower plates. Similarly to the left fore leg, the other legs, namely, a right fore leg, a left hind leg and a right hind leg, are also formed in a box-like structure enclosed by upper and lower plates and front and rear plates on four sides.
In the case of the truck frame of the prior art mentioned above, each one of the four legs of a truck frame is fabricated from four separate leg-forming plate members, that is to say, from upper, lower, front and rear plates. Therefore, at the time of forming and assembling each leg, it has been required to cut the upper, lower, front and rear plates according to the shape of the leg to be formed, and to weld the front and rear plates to the front and rear sides of the upper and lower plates on an assembling site. These cutting and welding operations are required for each one of the four legs, and of course reflected by a high truck frame production cost.
Further, at the time of welding to the side frames the four legs which are provided integrally with the center frame as mentioned above, it is also required to cut out a precision joint portion at the outer end of each leg, which will butt against a joint surface on the side of a side frame in a gapless state.
A great problem in this regard has been that it is very likely for machining errors to occur to a joint portion which is formed at the outer end of each leg by an on-site cutting operation, and the joint strength of the respective legs with a corresponding side frame is often impaired by existence of gap spaces or interstices in the butt-joined portions between the legs and the side frames.